MRI of the Spine: A Guide for Orthopedic Surgeons

By William B. Morrison and Adam E. Flanders

Utilizing plentiful radiological images to illustrate each topic, this text is a comprehensive and descriptive review of magnetic resonance imaging (MRI) interpretation for the spine, emphasizing standardized nomenclature and grading schemes. The book begins with current MR imaging protocols, including indication, sequencing and advanced imaging techniques, and a review of the relevant anatomy of the spine and its anomalies. Subsequent chapters encompass topics of trauma, degenerative disease, infection, inflammatory disease, as well as neoplastic and metabolic disease. Spinal cord and dural lesions will also be presented, with additional chapters dedicated to MRI evaluation of the post-operative patient. The format is reader-friendly, utilizing an efficient presentation of the essential principles and important findings on MR images of the spine, with a wealth of high-quality figures, graphics and tables for differential diagnosis as well as tips and tricks from experts in the field. 

Presenting the most up-to-date protocols and suggested interpretations, MRI of the Spine will be a solid reference for orthopedic surgeons, sports medicine specialists, neurosurgeons, radiologists and all clinicians and support staff caring for the spine.

• Language: English
• Publisher: Springer
• Release date: May 22, 2020
• ISBN: 9783030436278

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© Springer Nature Switzerland AG 2020

W. B. Morrison et al. (eds.)MRI of the Spinehttps://doi.org/10.1007/978-3-030-43627-8_1

1. MRI Protocol

Vishal Desai¹   and Jehan Ghany¹  

(1)

Thomas Jefferson University, Philadelphia, PA, USA

Vishal Desai (Corresponding author)

Email: [e-mail]

Jehan Ghany

Email: [e-mail]

Keywords

MRIPhysicsProtocolGadoliniumSequences

Introduction

Magnetic resonance imaging (MRI) is the mainstay for noninvasive evaluation of the spine, providing detailed anatomical assessment and excellent sensitivity for pathology, including degenerative disc disease, tumors, infection, bone marrow processes, spinal cord abnormalities, traumatic injuries, and compression fractures. Unlike other imaging modalities, MRI can evaluate the spinal cord, meninges, cerebrospinal fluid, marrow, and supporting structures in one routine study.

Advanced MR imaging can be obtained with additional sequences and/or with intravenous contrast to gather more information, help with troubleshooting, or assist in evaluating patients with prior spinal surgery with or without hardware. The high yield of MRI and the lack of ionizing radiation make it the imaging modality of choice for the spine in nearly all populations and for most indications.

MRI Physics

To get a better understanding of the commonly performed MRI sequences and what information can be extracted from each, an overview of MRI physics is helpful [1]. MRI utilizes the body’s natural magnetic properties for imaging, specifically the hydrogen nucleus due to its prevalence throughout the body in water and fat.

Magnetic Field

Hydrogen protons contain a net positive charge, providing them with their own magnetic spins and a local magnetic field . With the patient in the MRI scanner, a uniform magnetic field is applied to the protons in the slice or slab of interest, causing the randomly oriented protons to now align parallel to the external magnetic field, precess at a certain frequency, and contain a net magnetization in the longitudinal direction (Fig. 1.1).

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Fig. 1.1

With the application of an external magnetic field, the previously randomly oriented protons are now aligned and have a net magnetization

RF Pulse

Next, a radiofrequency (RF) pulse or series of RF pulses is applied to the protons, dependent on the sequence and information desired. The energy from the RF pulse is absorbed by the protons, causing the net magnetization to tilt away from the longitudinal direction (Fig. 1.2). Again, the extent of this tilt depends on the RF pulse and can be manipulated to obtain different tissue characteristics.

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Fig. 1.2

After the protons are aligned by the external magnetic field (a), RF energy is applied by the scanner to tilt the protons into the transverse plane (b). As the protons move back to their lower energy state, the time to decay in the transverse plane and the time to relax to the longitudinal direction are related to the property of the tissues – T2 and T1, respectively (c, d). This provides important imaging characteristics

Relaxation

As with any structure with high energy, there is a natural tendency toward a lower energy state. With MRI, this means the protons which were tilted and rotated by the RF pulse (high energy state) will realign with the magnetic field (low energy state). The time taken for a particular tissue to relax in the longitudinal direction is referred as the T1 relaxation rate. The time taken for transverse magnetization to decay to 37% of its initial value is referred to as the T2 relaxation rate (Fig. 1.2). Both T1 and T2 relaxation occur simultaneously based on the intrinsic properties of the excited tissue and its local environment.

As the tissues relax toward the lower energy states, RF energy is emitted and can be measured. The time from the delivery of the RF pulse and peak of the signal emitted (also known as an echo) is referred to as the time to echo (TE). The time interval between each excitation pulse is referred to as the repetition time (TR). These parameters (time, duration, and sequence of RF pulses) can be manipulated to create different types of images to characterize different structures and help answer the clinical question.

Image Formation

The last step is to translate the received signal into an image. The signal is received as frequency information, encoded with location information and intensity determined by the tissue characteristics and sequence parameters. Through Fourier transformation , this frequency information is converted into shades of gray in a matrix of pixels , which forms an image.

Image Quality

An optimal imaging protocol considers the clinical question and tailors the examination to answer it, with high yield sequences acquired in the minimum amount of time and with excellent quality to ensure diagnostic adequacy. Quality is determined by image resolution, image contrast, signal-to-noise ratio, and lack of artifacts. Fine-tuning imaging protocols are required to find the appropriate balance between resolution, contrast, signal, noise, and overall study time.

Resolution

Image resolution determines the ability to evaluate small structures or pathologies and can be regarded as the level of detail in the scan. A high-resolution image is able to distinguish between adjacent structures, whereas a low-resolution image would blur them together (Fig. 1.3). This depends on the size of the image pixel (or voxel for 3D sequences), which in turn depends on the matrix size, field of view, and slice thickness.

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Fig. 1.3

An example of a high-resolution MRI image (a) compared to a low-resolution image (b), in which discerning small structures is difficult

Increasing the matrix size increases the total number of pixels/voxels, which means a higher-resolution image but at the cost of more time and with less signal in each voxel. A larger field of view may be required to image the entire region of interest (such as the thoracic spine), but that means that each voxel now contains more area, decreasing spatial resolution. Similarly, increasing slice thickness covers more area, decreasing spatial resolution.

Signal-to-Noise Ratio

For any imaging modality, the goal is to obtain the highest amount of signal with the least amount of noise. For MRI, this often comes at the expense of spatial resolution, as more signal is obtained when the voxel size is larger (increased slice thickness, smaller matrix, and larger field of view). Additional strategies to increase signal and decrease noise without affecting spatial resolution include increasing the number of excitations (NEX) and utilizing RF coils. By increasing the NEX, more signal is received per acquisition and allows for averaging of the signal for a higher-quality, less noisy image.

Proper coil selection is a crucial but often overlooked concept in MR imaging. There are a wide variety and multiple configurations of coils available, including several optimized for spinal imaging. The coil should sit closely against the area of interest and should be the smallest size possible. RF coils serve as the antenna for the MR scanner. By optimizing the shape, size, and location of this antenna, the signal received back from the body part being imaged is greater, improving image quality.

Contrast

In order to detect pathology, the image must be able to display a difference, or contrast, in signal intensity between normal and abnormal tissue. MRI inherently has high contrast sensitivity and is adept at demonstrating differences in tissues in the body, both anatomy and pathology (Fig. 1.4). Even though MRI has high contrast at baseline, the parameters must be optimized for each examination. By adjusting the TR and TE, the images can highlight tissue differences on T1-weighted, T2-weighted, and proton density sequences. More advanced sequences can evaluate and display other tissue characteristics if desired.

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Fig. 1.4

An example of T2-weighted sequence demonstrating the excellent contrast resolution of MRI, with a clear distinction between CSF, fat, and soft tissues

Field Strength

Besides intrinsic tissue contrast, several external factors can impact imaging quality, with one of the major ones being the primary property of the scanner – magnetic field strength. Magnetic field strength varies from 0.2 Tesla to 3 Tesla for clinical applications, with 1.5 Tesla the most commonly available and most commonly used. The higher the field strength, the better the contrast, the higher the resolution, and the higher the signal-to-noise ratio. This means that a stronger magnet improves overall image quality and thus diagnostic ability.

MRI scanners are available in both open and closed configurations. Open MRI scanners often have a lower field strength and several limitations, including a longer scan time which can predispose to motion artifact, poorer fat suppression, and a wider field of view to collect more signal. These limitations can contribute to lower-image quality compared to traditional scanners. As a result, open scanners should be reserved for claustrophobic and obese patients who may not fit in or cannot tolerate a closed system.

Artifacts

There are numerous artifacts that can impact image quality on MRI, which can be attributable to MR hardware and software, shielding of the MRI scanner room and magnetic field inhomogeneities, tissue heterogeneity, foreign bodies, patient motion, and physiologic motion [2] (Fig. 1.5).

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Fig. 1.5

(a) Motion artifact from the patient moving during image acquisition introduces blur and artifactual lines on the image. (b) Susceptibility artifact from dental hardware limits evaluation of the upper cervical spine

Based on the type of artifact, solutions exist to remove or at least partially correct for the artifact to improve overall image quality. Artifacts could contribute to confusing imaging findings (pseudo-masses) or preclude diagnostic evaluation (extensive susceptibility artifact from hardware). While a full discussion on MRI artifacts is beyond the scope of this chapter, recognizing that they exist and factors contributing to them can be helpful in image optimization and study interpretation.

Routine Sequences

The routine imaging protocol for a spine MRI should include a T1-weighted sequence (excellent for evaluating anatomy and bone marrow), a T2-weighted sequence (for anatomy and pathology), and a fluid-sensitive sequence (excellent for detecting pathology). At the minimum, a spine protocol should contain sequences in both the sagittal and axial plane.

T1-Weighted Sequence

Sequences with T1 weighting evaluate differences in the T1 contrast of tissues. After excitation, tissues relax back to equilibrium at different rates based on its intrinsic characteristics. Relaxation in the longitudinal direction provides the T1 property of the tissue.

Physics

In general, T1-weighted imaging utilizes a short TR and short TE to maximize tissue contrast (Fig. 1.6). Fat, for instance, rapidly realigns from the longitudinal direction back to equilibrium, yielding a higher signal and thus appearing bright, or hyperintense, on T1-weighted imaging. Conversely, water realigns slowly from the longitudinal direction, yielding less signal and thus appearing dark, or hypointense, on T1-weighted imaging. Without a short TR, all of the protons would relax back to equilibrium yielding an image with equal intensity for all tissues; thus, a short TR is required for T1-weighted imaging to achieve tissue contrast.

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Fig. 1.6

A short TR and short TE maximize T1 tissue contrast

Practical Application

This sequence is universally utilized in nearly all MR imaging protocols, in part due to its very high signal-to-noise ratio, and is primarily used for evaluation of anatomy [3]. Due to the bright appearance of fat on this sequence, T1-weighted imaging is especially useful for evaluation of normal bone marrow in the spine. Normal marrow in adults is predominantly fatty (yellow marrow) and should appear hyperintense. Pathologic marrow, which can be seen with lymphoma, leukemia, metastases, infection, and other infiltrative process, results in hypointense marrow. A rule of thumb for spinal imaging is that normal bone marrow on T1-weighted imaging should be brighter than the disc space; otherwise this should raise a red flag for an infiltrative process (Fig. 1.7).

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Fig. 1.7

(a) T1-weighted image demonstrating normal fatty bone marrow. (b) T1-weighted image in a different patient shows replacement of the normal fatty marrow. The vertebral body appears darker than the adjacent disk, a sign of a pathologic marrow process, which in this case was lymphoma

Besides fat, other structures that appear hyperintense on T1-weighted imaging include methemoglobin, melanin, slow-flowing blood, and proteinaceous fluid. T1-weighted imaging is also utilized after administration of IV contrast agents (such as gadolinium), which will be discussed separately.

T2-Weighted Sequence

T2-weighted imaging is another mandatory sequence for nearly all MR imaging and is useful for evaluating both anatomy and pathology. This sequence evaluates differences in the T2 contrast of tissues, another inherent property (although it can be somewhat impacted by magnetic field inhomogeneities). Whereas T1-weighted imaging relies on relaxation in the longitudinal direction, T2-weighted imaging relies on decay in the transverse direction [3].

Physics

For T2-weighted imaging, long TR and TE times are required to accentuate differences in tissue contrast (Fig. 1.8). Water has a long T2 relaxation time, resulting in a hyperintense appearance on a T2-weighted image. Fat also appears bright on T2-weighted imaging.

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Fig. 1.8

A long TR and long TE maximize T2 tissue contrast

Practical Applications

For spine imaging, T2-weighted sequences allow excellent visualization of the cerebrospinal fluid (CSF) due to its high water content. As a result, loss of normal CSF space, such as in the case of a disc herniation narrowing the central canal or neural foramen, will be readily apparent on this sequence (Fig. 1.9).

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Fig. 1.9

T2-weighted imaging is excellent for evaluation of degenerative disc disease due to contrast in disc material, CSF, and nerve roots. There is a large disc extrusion seen on sagittal (a) and axial (b) T2-weighted sequences in this patient at the L3-L4 level

Most pathologic conditions – tumors, infections, inflammatory process – have a component of edema and as a result appear hyperintense on T2-weighted imaging. For instance, an epidural collection will generally appear bright or intermediate in signal on this sequence, perhaps with heterogeneity in the case of infection or blood products.

Fluid-Sensitive Sequence

While routine T2-weighted image will demonstrate fluid (and, as a result, most pathology) as hyperintense, fat also appears hyperintense which can make identifying and characterizing the abnormality difficult. Thus to get a truly fluid-sensitive sequence, different techniques can be applied to suppress the fat and highlight the presence of edema and pathologic tissues (Fig. 1.10).

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Fig. 1.10

Fluid-sensitive imaging (STIR, in this example) is helpful to highlight pathology, particularly edema, which in this case makes the pars interarticularis stress fracture readily apparent (a) compared to non-fat-suppressed imaging (b)

Fat-Saturated T2-Weighted Sequence

The most commonly used technique to achieve a fluid-sensitive image is frequency-selective fat suppression. This method applies an RF pulse to the slice at the same resonance frequency of lipids to saturate all tissues with fat followed by a gradient pulse to nullify any signal from the lipid [4].

The advantage of this technique is that it can be applied to any sequence to suppress fat, including post-contrast imaging. However, it is highly susceptible to inhomogeneities in the magnetic field, which can lead to failure in fat suppression. Fat saturation techniques should not be used when metallic hardware is present in the area of interest (such as fixation devices in the spine) due to the resulting artifact.

STIR Sequence

An alternative and also commonly used sequence for fluid-sensitive imaging of the spine is short tau inversion recovery, or STIR. Since fat has such a short T1 relaxation time, shorter than most other tissues, its signal can be selectively nullified without impacting other tissues through the use of RF pulses [4].

STIR imaging is significantly less susceptible to inhomogeneities in the magnetic field, leading to more uniform fat suppression. This is the preferred method for fluid-sensitive imaging in the presence of hardware. The only disadvantage is that the fat suppression is not selective for lipids but applies to any tissues with short T1, such as melanin, mucus, and, of particular interest in spine pathology, methemoglobin. For the same reason, STIR should not be used for post-contrast imaging, as the T1 relaxation properties of contrast are similar to fat and would result in signal loss of both.

Imaging Planes

Routine imaging of the spine should include T1-weighted, T2-weighted, and fluid sensitive (fat-saturated or STIR) sequences in the sagittal imaging plane. This combination allows for evaluation of the bone marrow, CSF, and for most pathologies. Axial imaging with at least T2-weighting should also be performed to allow for evaluation in more than one plane. Often, a T2-weighted sequence in an axial plane relative to the disc space is included to improve evaluation of degenerative disc disease (Fig. 1.11).

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Fig. 1.11

Routine spinal imaging should include an axial T2-weighted sequence oriented to the disc spaces to improve evaluation of disc disease. The sagittal sequence demonstrating the correct plane selection (a) and resultant axial sequence through the disc space (b) are shown

Contrast

Contrast-enhanced MRI may be used to help identify and further characterize pathology, particularly neoplastic and inflammatory processes. Gadolinium-based contrast agents are the most commonly utilized and, for spinal imaging, are administered intravenously.

Physics

Gadolinium contrast is a paramagnetic agent, meaning that it contains unpaired electrons which results in a local magnetic field wherever the contrast is present. Pathologic conditions tend to have hypervascularity and as a result accumulate contrast after intravenous administration, referred to as enhancement. The presence of gadolinium in these tissues alters the local magnetic field, resulting in T1 shortening or hyperintense appearance on T1-weighted imaging [5].

Practical Applications

Due to its impact on T1 imaging properties , the sequence of choice to complement contrast administration is a T1-weighted fat-saturated sequence. This allows enhancing pathology to appear bright while suppressing signal from lipids to avoid confusion.

The presence or absence of enhancement in addition to the pattern of enhancement can help determine benignity versus malignancy for certain lesions and help narrow the differential diagnosis for others. In spinal imaging, contrast is utilized for characterizing masses (in the spinal canal, soft tissues, or marrow), evaluating infection/inflammation, and assessing recurrent disc herniations or complications in the postoperative spine (Fig. 1.12).

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Fig. 1.12

Post-contrast axial (a) and sagittal (b) imaging of the lumbar spine demonstrates a large heterogeneous, enhancing mass inseparable from the enhancing left L5 nerve root, which was biopsy proven to be a nerve sheath tumor

Contrast Safety

Severe adverse reactions to gadolinium-based contrast agents , such as anaphylaxis, are relatively rare (1 in 100,000). Most reactions are self-limiting and include headaches, injection site pain, and nausea. Besides a history of prior severe adverse reaction, the only other relative contraindications include pregnancy and renal failure. Gadolinium-based contrast agents have been linked to nephrogenic systemic fibrosis in patients with renal failure and should be used cautiously in patients with eGFR <30 mL/min/1.73 m² [6].

Non-routine Sequences

Based on patient history, suspected pathology, or previous imaging findings, additional imaging sequences may be added to the routine protocol for better delineation and problem solving.

Proton Density Sequence

In proton density (PD)-weighted images , the signal intensity directly corresponds to the density of hydrogen atoms (protons) in the tissues. PD is considered an intermediate sequence, with a short TE and relatively long TR, producing imaging features of both T1 and T2 [3]. This sequence has a high signal-to-noise ratio, greater than both T1- and T2-weighted sequences. This sequence is commonly added to spinal imaging protocols to evaluate for demyelination, such as in multiple sclerosis, and has been shown to be more sensitive in that regard than T2-weighted imaging (Fig. 1.13) [7].

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Fig. 1.13

Proton density imaging improves detection of demyelinating lesions in the spinal cord, as seen at the C2 and C3-C4 levels in this patient with multiple sclerosis

In-Phase and Out-of-Phase Imaging

In-phase and out-of-phase sequences , also referred to as chemical shift imaging , are paired sequences that assess for the presence of intralesional microscopic fat, a feature commonly regarded as a sign of benignity. Due to slight differences in resonance frequencies of protons in water versus fat, their spins are routinely in phase and out of phase. These differences can be exploited by acquiring images at the same TR but differing TE, such that when the spins are in phase the signal from fat and water within a voxel is additive and when out of phase is cancelled out.

As a result, if the tissue or lesion of interest contains both fat and water in the same voxel (such as edema or hematopoietic bone marrow), then there will be loss of signal (or dropout) on out-of-phase imaging relative to the in-phase sequence. If there is no signal dropout, then microscopic fat is not present suggesting neoplasia. This is particularly useful for osseous lesions in the spine, including evaluation of vertebral body compression fractures, since normal bone marrow should contain varying amounts of fat (Fig. 1.14). The dropout can be quantified using a region of interest (ROI) tool, found in nearly all image viewers, with a 20% drop from in-phase to out-of-phase commonly used as cutoff to maximize sensitivity and specificity [8].

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Fig. 1.14

In-phase (a) and out-of-phase (b) imaging is helpful to identify microscopic fat, a sign suggestive of a benign process. In this example, there is no drop in signal on out-of-phase imaging within the L4 lesion, which means there is no intralesional microscopic fat. This lesion was therefore deemed indeterminate and biopsy yielded a metastatic lesion

False positives can occur with chemical shift imaging, such as in the presence of acute blood products, marrow fibrosis, sclerotic metastases, and fat-containing metastases which can all show loss of signal. Correlation with other sequences can help reduce misinterpretation.

Diffusion-Weighted Imaging

Diffusion-weighted imaging (DWI) is a powerful imaging sequence sensitized to the movement (diffusion) of water molecules. Any changes in the tissues or local environment that produce a barrier, such as ischemia, abscess, or tumor, will restrict diffusion. This can be utilized for MR imaging to help identify and characterize lesions. Gradient pulses are applied to the tissues of interest; protons that have not moved or have only minimally moved in between the pulses (restricted diffusion) will demonstrate the highest signal on the image.

DWI has revolutionized imaging of the brain for stroke and tumor evaluation and has more recently become routine for breast and body MRI. It is used much less frequently in spinal imaging due to artifact from the heterogeneous, complex anatomy (air, fluid, osseous, and soft tissues are all adjacent structures), motion artifact from breathing, and the relatively small size of the spinal cord for cord lesion characterization.

However, techniques to reduce these artifacts and obtain helpful diagnostic imaging have been created, and this sequence should be considered for troubleshooting indeterminate findings on routine imaging. Newer studies have shown DWI of the spine to be a useful adjunct for characterizing spinal cord lesions (infarcts, demyelination, myelomalacia, and tumors) in addition to intradural, epidural, and osseous lesions (Fig. 1.15) [9].

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Fig. 1.15

Diffusion-weighted imaging (a) demonstrates increased signal within the expanded cervical cord in this young patient with sudden onset of paralysis, with low signal on the ADC map (b), highly suggestive of a cord infarct

Advanced MRI Sequences

Dynamic Contrast-Enhanced Imaging

Dynamic contrast-enhanced (DCE) imaging acquires a series of MR images in succession after administration of intravenous contrast. This allows for characterization and quantification of the microvascular environment in the area of interest or wash-in and wash-out features of the lesion. A common feature among nearly all malignant processes is neoangiogenesis brought on by secretion of growth factors. Thus, DCE can help detect benign versus pathologic compression fractures, hypovascular and hypervascular masses, and the presence of residual or recurrent tumor post-treatment [10, 11].

Magnetic Resonance Angiography

Utilizing flow-related differences or contrast enhancement of vessels, magnetic resonance angiography (MRA) can help evaluate arteries and veins in and around the spine. This can assist in not only identifying dural arteriovenous fistulas and other vascular malformations but also in characterizing feeding and draining vessels (Fig. 1.16) [12–14]. Additionally, MRA can play an important role in preoperative planning to help identify the location of the artery of Adamkiewicz, critical spinal arteries, and veins to avoid inadvertent neurological damage.

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Fig. 1.16

Multiple flow voids are seen on T2-weighted imaging about the cord, a common finding for vascular lesions/anomalies (a). Magnetic resonance angiography confirms a dural arteriovenous fistula and identifies a possible feeding vessel (b)

Metal Artifact Reduction Sequence

For patients with spinal hardware, the MRI study must be optimized to minimize artifact from the hardware. This can be achieved using the methods discussed previously – avoid fat-saturation sequences, utilize STIR imaging, use low field strength magnets, obtain thinner slices, and adjust specific scan parameters such as bandwidth and matrix size. In addition, most vendors also include sequences with proprietary techniques to reduce metal artifact, such as MAVRIC (GE) and SEMAC (Siemens).

Diffusion Tensor Imaging

Diffusion tensor imaging (DTI) is an extension of DWI, relying on movement of water molecules. DTI allows for detailed imaging of white matter tracts, as diffusion generally occurs in the path of least resistance (along the tracts, rather than perpendicular to them). DTI may be used to evaluate for cord damage in traumatic injury or in the integrity of and involvement of white matter in neoplastic processes [15, 16].

Functional MRI

Functional MRI (fMRI) , originally utilized for brain imaging, has been adapted for use in the spine. Spinal fMRI depicts neural activity through changes in blood flow and blood oxygen levels about active gray matter. While still predominantly used for research purposes, fMRI clinically can be helpful in assessing areas of preserved and impaired function in spinal cord injury patients [17].

PET-MRI

While MRI is the imaging test of choice